In a closing method, a work piece constituted by a metal pipe material is rotated and a die is pressed against the work piece while the work piece is heated. Thus, the work piece undergoes plastic deformation as it gradually approaches the die, and as a result, an end portion of the work piece is formed into a closed bottom portion.
This closing method is disclosed in JP2002-153930.
The work piece formed into a closed-end shape through implementation of the closing operation is press-molded by a press-molding device. A conventional press-molding device comprises an inner die which is inserted into the inside end of the closed-end work piece, and an outer die disposed on the outside of the work piece. The inner die is driven by a hydraulic cylinder such that the bottom portion of the work piece is compressed between the inner die and the outer die.
In a conventional press-molding device, however, if the work piece moves when the inner die is inserted into the inside end of the work piece, the inner die may impinge on and damage the inner peripheral surface of the work piece.
To prevent this, the outer peripheral surface of the work piece may be gripped when the inner die is inserted into the inside end of the work piece so that the work piece does not move. However, when the outer peripheral surface of the work piece is gripped so that the work piece does not move, the work piece is unable to move in the axial direction thereof when the work piece is compressed between the inner die and outer die, and as a result, press-molding of the work piece cannot be performed smoothly.
Furthermore, a conventional press-molding device is structured such that the inner die is driven by a hydraulic cylinder and thereby inserted into the inside end of the work piece. As a result, the inner die cannot be moved quickly, leading to an increase in the tact time for press-molding a single work piece.
It is therefore an object of this invention to provide a press-molding method and a press-molding device for press-molding a work piece formed into a closed-end shape smoothly without damaging an inner peripheral surface of the work piece.